Congenital diaphragmatic defects are common birth defects with high morbidity and mortality secondary to associated pulmonary hypoplasia. Despite the significant impact that these defects have, the molecular mechanisms underlying their development are not understood. We identified a hypomorphic mutation in the mouse gene, Fog2 that causes a diaphragmatic defect with primary pulmonary hypoplasia and a de novo FOG2 mutation in a baby with a posterior diaphragmatic defect and pulmonary hypoplasia. FOG2 is thus the first gene implicated in the pathogenesis of non-syndromic congenital diaphragmatic defects, and its necessity for pulmonary development validates the hypothesis that neonates with congenital diaphragmatic defects may also have primary pulmonary abnormalities. We found that Fog2 dependent lobar development is mediated by a Fog2-Gata4 interaction. Gata4 is also implicated in diaphragm development in the mouse, and is a candidate gene for human diaphragmatic defects based on its location in a Congenital Diaphragmatic Hernia (CDH) cytogenetic hot spot. The purpose of this proposal is to identify pathway genes and mechanisms of Fog2-Gata4 mediated lung and diaphragm development. In Specific Aim 1, genes that share expression patterns with Fog2 and Gata4 in the early branching lung will be investigated, and genes will be identified that are modulated by Fog2-Gata4 interactions in the lung at the time of lobar budding. Specific Aim 2 will evaluate the role of retinoic acid signaling in Fog2 and Fog2-Gata4 mediated development, as retinoic acid plays a role in both diaphragm and lung development, and retinoic acid receptors interact with Fog2. In Specific Aim 3, it will be determined whether Fog2 and Gata4 are needed in posterior mesenchymal tissue for normal development. Since Fog2 and Gata4 are required for nomal human lung and diaphragm development and both are CDH candidate genes, this proposal will identify the genetic pathways that control this development leading to a better understanding of the pathogenesis of human diaphragmatic defects.